(1) Field of the Invention
The present invention relates to the production of sheet material and particularly to the manufacture of such material having characteristics which render it suitable for application in or as electrodes for use in electrochemical devices such as fuel cells. More specifically, the present invention is directed to catalyst containing polytetrafluoroethylene electrodes which exhibit greater resistance to flooding in open circuit air potentials than has characterized previous devices of similar character. Accordingly, the general objects of the present invention are to provide novel and improved methods and articles of such character.
(2) Description of the Prior Art
While not limited thereto in its utility, the technique of the present invention is particularly well suited for use in the production of fuel cell electrodes. As is well known in the art, fuel cells produce, from a fuel and oxidant, electrical energy. In its most simplified form, the fuel cell consists of a housing, an oxidizing electrode, a fuel electrode and an electrolyte disposed between and in contact with the two electrodes. In operation, it is necessary that the fuel and oxidant contact a first surface of their respective electrodes, where a process of adsorption and de-adsorption occurs, leaving the electrodes electrically charged. The second surfaces of the electrodes; i.e., the facing surfaces; will be in contact with the electrolyte which may be trapped in a porous sheet known in the art as the matrix. Depending upon the nature of the electrolyte, ions are transferred through the electrolyte from the anode to the cathode or from the cathode to the anode thereby enabling the withdrawal of electrical current from the cell.
In order to obtain optimum performance from a fuel cell, the electrodes must insure optimum contact of reactant gas, electrolyte and electrode reaction sites for minimum electrode polarization. The electrodes will include a catalyst and in order to get electrolyte to the reaction sites the catalyst must be wetted by the electrolyte. In order to get the reactant or oxidant gas to the reaction sites it is also necessary to have gas channels or pores in the electrodes. These gas channels must not become filled with electrolyte, in which case gas transfer would be blocked, and must thus by hydrophobic. The electrodes must, additionally, have sufficient mechanical strength to withstand the operating environment without fracture and have a low bubble pressure.
In the interest of providing electrodes having the above briefly described characteristics, resort has typically been had to the formation of bi-layer electrode structures. In such bi-layer electrodes the layer which is exposed to the reactant gas will typically be comprised of a "carbon paper" which has been treated so as to render it hydrophobic. As used herein the term "carbon paper" refers to a sheet material consisting of carbon fibers which have been bonded together to form a gas permeable paper-like structure having sufficient strength to function as a support plate for the electrode. The second layer, which is bonded to and supported by the "carbon paper" and which has its exposed face in contact with the electrolyte, is known as the catalytic layer. The catalytic layer, which is partially wettable to the electrolyte and is characterized by gas porosity, controls the electrolyte interface within the electrode and prevents flooding of the electrode which would result in damage and/or loss of power.
Because of its highly desirable chemical, electrical and mechanical properties, polytetrafluoroethylene (hereinafter PTFE) has long been recognized as a material which has great potential for use in fuel cell electrodes. In order to take advantage of the desirable properties of PTFE, the catalytic layer of an acid fuel cell may theoretically consist of a gas permeable layer formed from a dispersion of a catalyst and PTFE; the catalyst typically being a noble metal black such as platinum on carbon black. Prior art efforts to form such a PTFE based catalytic layer have relied upon the spherical particle shape of the PTFE to impart the requisite gas porosity to the layer. Thus, attempts have been made to rely upon a naturally formed or inherent porosity. In actual practice, because of the necessity of sintering the electrode to reduce pore size and thereby preclude flooding by the electrolyte, erratic results have been obtained. The poor results are believed to be caused by either the flowing of the PTFE and subsequent closing of the gas channels during sintering with a concomitant poor wetting of the catalyst by the electrolyte or by a tendency to flood resulting from insufficient sintering and concomitant excess electrolyte adsorption.